Cladistic and phenetic analyses of leaf and other morphological characters of Gunnera strongly support monophyly of the genus, with the Saxifragaceae s.str. as the closest sister group. This morphologically based phylogeny provides a more coherent understanding of the evolutionary history of Gunnera than do recent phylogenetic hypotheses based on genetic data sets with Myrothamnaceae as the sister group. Simple, crenate, palinactinodromously veined leaves lacking freely ending veinlets and tricolpate, tectate-perforate pollen with a reticulate exine indicate a shared ancestry. Within the genus Gunnera all six traditionally recognized subgenera are monophyletic, as supported by leaf architectural apomorphies. The monotypic subgenus Ostenigunnera is the sister group to the other five subgenera, which can be divided into two principal lineages. One lineage includes the subgenera Milligania and Misandra, characterized by a prostate stoloniferous habit with small, low-rank leaves and exclusively unisexual flowers, whereas the other lineage includes the subgenera Perpensum, Pseudo-Gunnera, and Panke, all of which possess at least some hermaphroditic flowers and larger, high-rank leaves. When the phylogeny of the subgenera is considered in light of biogeography and the fossil record, a number of cladogenetic events can be explained by continental vicariance in the Late Cretaceous. The African Perpensum became distinct from the other large-leafed lineage with the separation of the African continent ca. 90 Ma. The two small-leafed lineages, the subgenera Milligania and Misandra, split with the separation of New Zealand from Western Gondwana, about 80 Ma. Pseudo-Gunnera became isolated from Panke prior to this time, when Panke fossils occur in North America. Gunnera probably arose out of an early herbaceous radiation of tricolpate eudicots having close affinity to the basal Saxifragaceae, especially the genus Chrysosplenium.